This invention generally relates to internal combustion engines and more particularly to a combustion control system for such engines.
In recent years, much effort in the automobile and petroleum industries has been directed toward controlling the manner in which ignition, flame propagation, and combustion take place within each cylinder. For example, the use of tetraethyl lead additives to suppress detonation, combustion chamber design changes, swirl charges, automatic ignition timing controls, and high-energy spark devices, have all contributed to the control of combustion.
One of the most important recent changes in fuel composition has been the removal of lead-based anti-knock compounds to eliminate lead oxide emissions from the exhaust. Effective non-polluting substitutes for the lead compounds have not been found so the octane ratings of presently available "unleaded" fuels have been significantly reduced.
Cylinder detonation or "knock" has been a most difficult problem to deal with. When a fuel/air mixture in an engine cylinder is ignited by a spark plug, there is approximately a five and one-half (51/2) millisecond delay before a flame front is established. Thereafter the combustion wave front expands in a relatively uniform manner. Shortly, however, as heat and pressure increase and before the wave front reaches the opposite side of the cylinder wall, the unburned fuel ahead of the front suddenly explodes, creating sound waves that hammer against the cylinder and piston resulting in vibrations which are heard as "engine knock". The reason that the unburned fuel prematurely explodes is because it was raised to its ignition threshold by the elevated heat and pressure in advance of the combustion wave front. As engine load increases, the tendency of the fuel/air mixture to detonate also increases.
Detonation is a serious problem because it results in loss of engine power, a reduction in efficiency and an increase in engine operating temperature. If allowed to persist, it will ultimately damage or destroy the engine. The detonation problem was substantially solved in the 1930s by the discovery that the addition of certain lead compounds to gasoline materially reduced the tendency of certain radical molecules to prematurely explode. A radical molecule, such as hydroperoxide (HO2), is formed by the partial breakdown of larger molecules in the presence of air. Given enough time during flame propagation, these radicals congregate just ahead of the wave front and somewhere in the second half of the flame travel, become a direct cause of autoignition of the remaining unburned portion of the cylinder charge, resulting in detonation. The tetraethyl lead compounds previously contained in gasoline, would turn to lead oxide in the cylinder and act as a radical "trap". The combined effect of the removal of those lead compounds from modern fuels, and the use of higher air-to-fuel ratios has resurrected the problem.
One solution commonly employed in currently available engines, is to reduce the compression ratio. That solution is not satisfactory because it does not entirely solve the problem and reduces engine efficiency.
Another solution which has been suggested to produce more uniform and complete combustion of rarified fuel mixtures, involves the use of a so-called "stratified" charge in the combustion chamber. The cylinder of an engine designed to operate with a stratified charge normally includes a main cylinder in which the piston reciprocates and a smaller auxiliary chamber in communication with the upper portion of the main cylinder. In operation, a small quantity of relatively rich fuel mixture is placed in the auxiliary chamber near the time of ignition and is ignited by a spark plug located in the auxiliary chamber. The combustion wave front originates in the auxiliary chamber and spreads into the main chamber, causing a multipoint ignition of the leaner main fuel charge. While engines of this design have enjoyed limited success, they have not seen wide-spread commercial use because of their limited effectiveness.
A third solution has been the use of high-energy ignition (HEI) which is designed to "force" a high voltage current to cross the gap of the spark plug electrodes while the spark plug is operating in leaner fuel/air mixtures. Although successful to date, it does require the ignition system to operate within rather critical parameters.
Accordingly, it is an object of this invention to provide for a combustion control system which will significantly reduce the problem of detonation in engines operating with lower octane, lead-free fuels, and do it without any modification of engine designs.
Recent research has shown that octane, a gasoline fraction twice as large as butane, breaks in half to form additional butane during the combustion process. With an overabundance of butanes (with their fast flame speed) in the second half of the engine combustion cycle as a contributing factor in the cause of detonation, it becomes an object of this invention to "shift" most of the butanes contained in gasoline, using them to not only initiate combustion, but also to increase the flame propagation speed so that a normal flame front will reach the opposite side of the combustion chamber before detonation can "set up".
It is yet another object of this invention to provide for a combustion control system in which normally wasted engine heat is used to remove most of the butanes from the gasoline fuel supply just prior to its use in the engine.
Finally, it is a further object of this invention to provide for a combustion control system which includes a combined spark plug and precombustion chamber into which a butane/air mixture is injected and ignited in order to initiate and manage combustion wave propagation within the engine cylinder.